Dear Mitch (and Philip and Phil),
It is clear that I should give refmac a go with the non-detwinned F's and just the TWIN command.
Thank you for your suggestions,
Herman
-----Ursprüngliche Nachricht-----
Von: Miller, Mitchell D. [mailto:[log in to unmask]]
Gesendet: Donnerstag, 20. Juni 2013 16:18
An: Schreuder, Herman R&D/DE
Betreff: RE: Twinning problem
Hi Herman,
Have you considered the possibility of your crystals being tetartohedral twinned. That is more than one of the twin laws may apply to your crystals.
E.g. in P32 it is possible to have tetartohedral twinning which would have
4 twin domains - (h,k,l), (k,h,-l), (-h,-k,l) and (-k,-h,-l). Perfect tetartohedral twinning of P3 would merge in P622 and each twin domain would have a faction of 0.25.
We have had 2 cases like this (the first 2PRX was before there was support for this type of twinning except for in shelxl and we ended up with refined twin fractions of 0.38, 0.28, 0.19, 0.15 for the deposited crystal and a 2nd crystal that we did not deposit had twin fractions of 0.25, 0.27, 0.17, 0.31). The 2nd case we had was after support for twining (including tetartohedral twinning) was added to refmac (and I think phenix.refine can also handle this). For 2NUZ, it was P32 with refined twin fractions of 0.25, 0.27, 0.17, 0.31.
Pietro Roversi wrote a review of tetartohedral twinning for the CCP4 proceedings issues of acta D http://dx.doi.org/10.1107/S0907444912006737
I would try refinement with refmac using the original (non-detwinned F's) with just the TWIN command to see if it ends up keeping twin fractions for all 3 operators (4 domains) -- especially with crystals 1 and 3 which appear to have the largest estimates of the other twin fractions.
Regards,
Mitch
==========================================
Mitchell Miller, Ph.D.
Joint Center for Structural Genomics
Stanford Synchrotron Radiation Lightsource
2575 Sand Hill Rd -- SLAC MS 99
Menlo Park, CA 94025
Phone: 1-650-926-5036
FAX: 1-650-926-3292
-----Original Message-----
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of [log in to unmask]
Sent: Thursday, June 20, 2013 6:47 AM
To: [log in to unmask]
Subject: [ccp4bb] Twinning problem
Dear Bulletin Board,
Prodded by pdb annotators, which are very hesitant to accept coordinate files when their Rfactor does not correspond with our Rfactor, I had a look again into some old data sets, which I suspect are twinned. Below are the results of some twinning tests with the Detwin program (top value: all reflections, lower value: reflections > Nsig*obs (whatever that may mean). The space group is P32, the resolution is 2.3 - 2.6 Å and data are reasonable complete: 95 - 100%.
From the Detwin analysis, it seems that the crystals are twinned with twin operator k,h,-l with a twinning fraction of 0.3 for crystal 1, 0.15 for crystal 2 and 0.4 for crystal 3. Crystal 2 can be refined while ignoring twinning to get acceptable but not stellar R and Rfree values. However, when I try to detwin Fobs of e.g. crystal 1 (twinning fraction 0.3), R and Rfree values stay about the same, whatever twinning fraction I try. At the time, I used the CNS detwin_perfect protocol to detwin using Fcalcs, which brought the Rfactors in acceptable range, but I do not feel that was the perfect solution. Ignoring twinning on e.g. crystal 1 produces an Rfactor of 22% and an Rfree of 29%
Do you have any idea what could be going on?
Thank you for your help!
Herman
Crystal 1:
operator -h,-k,l
Suggests Twinning factor (0.5-H): 0.113
Suggests Twinning factor (0.5-H): 0.147
operator: k,h,-l
Suggests Twinning factor (0.5-H): 0.277
Suggests Twinning factor (0.5-H): 0.323
operator -k,-h,-l
Suggests Twinning factor (0.5-H): 0.101
Suggests Twinning factor (0.5-H): 0.134
Crystal 2:
operator -h,-k,l
Suggests Twinning factor (0.5-H): 0.077
Suggests Twinning factor (0.5-H): 0.108
operator: k,h,-l
Suggests Twinning factor (0.5-H): 0.126
Suggests Twinning factor (0.5-H): 0.161
operator -k,-h,-l
Suggests Twinning factor (0.5-H): 0.072
Suggests Twinning factor (0.5-H): 0.106
Crystal 3:
operator -h,-k,l
Suggests Twinning factor (0.5-H): 0.123
Suggests Twinning factor (0.5-H): 0.149
operator: k,h,-l
Suggests Twinning factor (0.5-H): 0.393
Suggests Twinning factor (0.5-H): 0.433
operator -k,-h,-l
Suggests Twinning factor (0.5-H): 0.110
Suggests Twinning factor (0.5-H): 0.133
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